THE FACTORS WHICH DETERMINE SEX 673 



Payne 1 has recently shown that in GII/I/H/H* &-i///it/is then- are 

 two sorts of spermatozoa possessing respectively sixteen and ninet'-m 

 chromosomes, whereas the eggs are uniform in containing nineteen 

 chromosomes. Furthermore, the females are believed to have three 

 more chromosomes than the males (i.e. thirty-eight as compared with 

 thirty-five). 



In another insect, Lyi/fii* A/V/-///-/X the male differs from the 

 female, not in having fewer chromosomes, but in the possession of a 

 pair of different sized chromosomes. The body-cells in the male 

 have twelve ordinary chromosomes and two sex chromosomes which 

 behave differently from the others. One of the sex chromosomes is 

 larger than the other, and has been called the X chromosome, tin- 

 second or smaller one being termed the Y chromosome. 



After synapsis, or the pairing of the chromosomes preparatory 

 to spermatogeuesis, there are six double chromosomes, and the two 

 sex chromosomes, X and Y, which remain separated. At the first 

 maturation division all the chromosomes divide, the two resulting 

 cells each having eight chromosomes, including the X and Y. At the 

 second division, however, although all the other chromosomes divide, 

 the X and Y chromosomes do not divide, but each passes into a 

 separate product of division, that is, into a separate spermatozoon. 

 Each spermatozoon, therefore, contains six ordinary chromosomes and 

 either an X or a Y chromosome. In the female body-cells there are 

 twelve ordinary chromosomes and two sex chromosomes which are 

 similar, X and X. Preparatory to oogenesis there are seven double 

 chromosomes, the X's uniting together just as the other chromosomes 

 unite in pairs. The polar bodies are formed, and reduction OMMIVS in 

 the usual way, each egg containing six ordinary chromosomes and 

 one X chromosome. The ova, therefore, are all similar, but those 

 fertilised by spermatozoa with X chromosomes become females, and 

 those fertilised by V -bearing spermatozoa become males.- 



The mode of sex inheritance displayed by /,//>/"">. is believed to 

 be characteristic of the very large class of organisms in which the 

 males are heterozygous as regards sex, and the females homozygous. :< 



tion with smear prejxirations instead of sections. tind im evidence of a 

 accessory chromosome in .\ .,.*,! f/-/.i/<X and l*-li?vr that tin- body described M 

 such by Wilson is a plasino-"m.- and not a chromosome. 



1 Payne, "On the Sexual iMMviences in the Chromosome <;n>ups in oWy ////.< 

 ,/, ffuK." liil. Hull., vol.xiv., UX)8. Corrensalso has shown that in som.- plants 

 there are two classes of male germ-cells, and that these an- produced in 

 niiml>ei i s (/)>'<' />'./ ///'//'//// Una l"< /'//,./,,,/ </. '/<.<<//// 7<^.< ttacn i-'n l''/-x 

 mil hi>l ' -,< /W<//' ://. Berlin, 1907). 



Morgan. //://(//>_,/././>'./, Nr\v York, I'-'l^. 



:t The term hrtn-o/y^ote has Ix-t-n ^i\<-}\ l>y llati-soii to offspring 

 from the union of dissimilar gami-t->. Such organism-., acionlin^ t" the 

 Mendelian theory, produce more than one soil of gann-t- s ( - t - p. L'OO . 

 Homozygotes ai-e formed by the union of similar Lranift-s. and jn'oilm-e L r ann-t-. 



22 



